1. Field of the Invention
The present invention relates to a polyurethane resin binder for magnetic recording mediums. More particularly the present invention relates to a polyurethane resin binder for magnetic recording mediums, used in a magnetic layer formed on a non-magnetic support, which comprises a specified polyurethane and is excellent in dispersion characteristics, electromagnetic transducing characteristics, running durability, and long-term storage durability.
2. Related Background Art
Generally, magnetic recording medium for use in audio apparatuses, video apparatuses, computers, and the like, (specifically those used for audio tapes, video tapes, floppy discs, computer data tapes and the like) are made by forming a magnetic layer by applying a magnetic paint containing magnetic powder, a binder and the like on a non-magnetic support such as a polyester film and drying it.
For the binder for forming the magnetic layer of the magnetic recording mediums, generally used are vinyl chloride resins, polyurethane resins, polyester resins, nitrocellulose resins, epoxy resins, and the like.
Recently, with the expansion of the application fields of magnetic recording mediums such as audio tapes, video tapes, computer data tapes, etc., various performance are demanded therefor. In particular, high reliability of the magnetic recording mediums are increasingly demanded.
More specifically, demanded particularly are adaptability to high-density recording, high-speed long-time running, and long-time driving under high temperature and high humidity conditions, long-term storability, and so on, in order to record and reproduce clear tone and image.
For realizing high-density recording, atomization and high-magnetization of the magnetic powder are intended, and furthermore, increase of packing density of the magnetic powder in the magnetic layer is more and more eagerly intended. However, the increase of the specific area of the magnetic powder resulting from the atomization, and increase of the cohesive power owing to the high-magnetization bring about disadvantages such that dispersion of the magnetic powder becomes so difficult that a conventional binder cannot give sufficient dispersibility and surface properties, and that the increased surface area raises the surface activity of the magnetic powder and affects severely the binder for dispersion of the magnetic powder to cause deterioration of the binder. It also makes difficult the increase of the packing density of the magnetic powder.
On the other hand, a magnetic recording medium is violently contacted with a recording head, rolls, etc. upon recording and reproducing, which may induce wearing or defoliation of the magnetic layer, causing decrease or variation of reproduction output, noise generation, increase of drop-out, increase of the friction coefficient, deffective running properties caused by soiling of the roll with dropped powder, squeeking of the tape, clogging of the magnetic head, or other disadvantageous phenomena.
Furthermore, the countermeasures are required against defoliation of the magnetic layer and sticking of the magnetic layer resulting from deterioration of the magnetic layer caused by the use under high temperature and high humidity conditions or a long-term of storage.
A polyurethane resin is proposed which has high durability useful for a binder system improved in resistance to scratching, wearing, high temperature, and high humidity, giving high-speed running durability, and improved long-term storability. However, the improvement in durability of recording mediums using a known binder is still unsatisfactory, and the binder has not satisfactory properties as a binder for atomized magnetic powder or highly magnetized magnetic powder because of the inferior dispersibility. Thus further improvement is desired.
As describe above, no binder has been developed which exhibits high durability and has sufficient dispersibility for atomized magnetic powder. Thus, a magnetic recording medium is demanded for, which has desired durability, magnetic properties and electromagnetic transducing characteristics. For this purpose, a binder for magnetic recording mediums is demanded, for which are superior in long-term running performance, high-temperature high-humidity long-term running performance, and long-term storage durability and have a greatly improved dispersibility of the magnetic powder and surface properties of the magnetic layer.